1. Field of the Invention
The present invention relates to an apparatus for producing high-purity, single-crystal semiconductor ingots, in particular of silicon, from a melt under protective gas. It furthermore relates to a process for the crucible pulling of high-purity, single-crystal silicon ingots using this apparatus.
2. The Prior Art
In the Czochralski crucible pulling of crystal ingots, in particular semiconductor ingots, the material provided for producing the melt is usually introduced into the melting crucible in lump form. The crucible temperature is then increased by heating until the melting point is reached and the crucible contents are gradually converted to the molten state. A seed crystal with the planned crystal orientation is then applied to the melt and is drawn out of the melt, in general while rotating crucible and crystal. The crucible pulling process is explained in detail, for example, in W. Zulehner and D. Huber, Czochralski-Grown Silicon, Crystals 8, Springer Verlag, Berlin-Heidelberg, 1982, and in the literature cited therein with particular attention being paid to the currently most important filed of application, namely, the crucible pulling of silicon single crystals.
To produce the melt, the melting crucible, which is composed, as a rule, of material which is substantially inert towards the melt, such as, for instance, quartz in the case of silicon or gallium arsenide, or iridium in the case of gallium gadolinium garnet, is first filled as substantially as possible with lumpy melt material. Then the temperature is raised to above the melting point, for example, by means of radiation or resistance heating, and the crucible contents are gradually caused to melt. Since, however, the lumpy material introduced does not permit a complete filling of the space in the melting crucible even with optimum adjustment of the particle limits, the amount of melt produced therefrom can, ultimately, only partly fill the crucible. In relation to the melt actually produced, therefore, over-dimensioned crucibles have to be provided, brought to the high working temperatures, and held there. This mismatch manifests itself all the more, the larger the ingots pulled, and, consequently, manifests itself in the amounts of melt required. For example, the common ingot diameters in the case of silicon are at present about 100 to 200 mm, with increases to about 300 mm even being under discussion. Added to this in the case of some materials such, as, in particular, silicon and germanium, is the appreciable volume contraction in some cases in the transition from the solid to the molten state.
In many cases, there has for this reason been a desire to improve the degree of filling of the crucible by adding further solid melting material to the melt, based upon lumpy material introduced after melting. For this purpose, polycrystalline ingot pieces are immersed as recharging material in the surface of the liquid melt by means of suitable holding devices as a rule before the start of the actual pulling process, and gradually melted until the desired melt level is reached.
In order to increase the amount of melting material introduced, and to operate the pulling process continuously instead of batchwise, a transition has been made to continuously replenishing melting material even during the crystallization process from a raw material container. Processes of this type are referred to as continuous Czochralski processes and are described, for instance, in G. Fiegl, Solid State Technology, August 1983, page 121. Two recharging possibilities are known in principle from the prior art and they differ in whether the melt material is added to the melt in the crucible as a liquid or as a solid. A continuous Czochralski pulling process with recharging of solid melt material is described in EP-A-0 170 856 and 0 245 510, and one with recharging of liquid melt material is described in U.S. Pat. No. 4,410,494.
One possibility of recharging a solid, which was described in EP-A-0 170 856, is to add solid granular material to the melt via an inlet tube. Since the melting process in this case first takes place in the melt from which the crystal is also pulled, the region in which the solid material is added to the melt is separated from the region of the crucible from which the single crystal is pulled, since disturbances in the single-crystal dislocation-free growth otherwise occur in the recharging as a result of thermal disturbances in the melt and of temperature variations. There has therefore, for example, been a desire to separate the crucible containing the melt material into an annular outer chamber in which the recharging material is melted and an inner chamber, in communication therewith, in which the crystal is pulled.
Another possibility of recharging solid melt material is described in the embodiment presented in EP-A-0 245 510. The melt material is recharged in this case by melting ingots of crystalline material to an increasing extent, while the same amount of material is simultaneously crystallized as a single crystal. In this process, the newly-formed melt first drips into a funnel-shaped crucible insert in order to avoid harmful convection flows which result in defects in the crystal growth.
However, this process has the disadvantage that suitable compact polycrystalline ingots can only be produced with difficulty in the required quality. In addition to the cost expenditure needed for this purpose, the disadvantage emerges that such ingots are particularly susceptible to chipping, particularly under the conditions prevailing in the pulling vessel, as a result of which unmelted chipped-off crystal parts fall into the actual melt during melting and disturb the crystallization process.
In both processes for recharging solid melt material, the risk associated with the introduction of solid material remains unchanged in that crystals which are not dislocation-free are pulled. If new polycrystalline material or granular material to be melted is introduced, this is, as a rule, associated with the introduction of very fine, mobile particles of semiconductor material. These particles are produced even during the introduction and melting of polycrystalline silicon blocks which do not, as a rule, melt at a constant rate, but shatter during melting. This fine material produced does not enter the interior of the melt (silicon melt is known to have a higher density than solid silicon). Instead, the finest dust particles float on the surface of the melt or are located in the gas space above the melt, and are incorporated into the single-crystal formed without having been previously melted, and this then results in growth defects. Even a subdivision of the crucible cannot prevent these disadvantages.
These problems are all the more strongly manifested, the greater the proportion of recharged melt is compared with the melt originally contained in the main crucible. In particular, in shallow crucibles, which contain, after all, a correspondingly lesser amount of melt, and in the pulling of crystals with large diameter or large length, the risk of pulling imperfect crystals as a result of the large amount of recharged material is considerably increased.
A more complicated solution described in U.S. Pat. No. 4,410,494 is to provide two separate crucibles connected by a heated feedline so that the recharged material can be melted in a separate second crucible and recharged into the main crucible through the feedline.
This process has the disadvantage that the second vessel needed for melting causes increased investment and operating costs. Attempts are therefore made to connect a plurality of pulling vessels to one recharging vessel. This results in long feedlines between the vessels, which have to be permanently heated in order to prevent blockage. When the recharging vessel is shut down, for example, for maintenance or refilling work, all the pulling vessels have to be shut down. Conversely, if a pulling vessel is opened, for example, to insert a seed crystal, the recharging vessels also have to be shut down. If the feedlines are only temporarily provided for recharging, this results in contamination risks and the advantage of the continuous process, i.e. uninterrupted recharging facility, is lost.
A further disadvantage in all the processes hitherto known is the lack of accuracy in the metered addition of dopant. In general, it is desirable to establish constant dopant concentrations within narrow limits for the entire length of the ingot during the pulling process. This is made difficult by the fact that the dopant becomes either enriched or depleted in the melt during the crystallization because of differing segregation coefficients. Dopant also has therefore to be recharged, as a rule, during the pulling process. As a rule, the dopant is therefore added to the solid recharging material (that is to say, granular material or a polycrystalline ingot to be melted) or it is added to the melt material at the outset in an additional crucible. A change in the dopant concentration during the crystallization process is consequently made considerably more difficult, since the previous processes did not provide any facility for correcting the dopant concentration in the recharging material even during the pulling process without the system having to be opened.